Synthetic aperture radar imaging provides the ability to form high-resolution images of a target terrain under varying conditions. To form images, RF energy is transmitted and collected throughout a flight path, the path comprising a “synthetic” aperture. The collected energy is stored as phase history data. Methods for forming an image from the collected data often use a focus target plane (FTP), an imaginary flat surface approximating the terrain being imaged. Portions of the imaged terrain located within the FTP may appear focused, and portions outside of the FTP may appear out-of-focus.
Forming an image from the collected data is a computationally intensive process. A generalized matched filter approach, which requires knowledge of the target elevation and does not rely on a single FTP, requires on the order of N^2 computations, where N is the number of resolution cells in the image. Other image formation operations, such as polar format processing and range migrations are able to form the image with a single FTP with on the order of N*log(N) computations by using a Fast Fourier Transformation. The images formed by these other operations, however, appear out-of-focus at portions of the terrain above or below the FTP. Some compensation for the out-of-focus portions can be made through the use of spatially-variant autofocus, which divides an image into a series of patches, raising or lowering the FTP on individual patches. Use of spatially-variant autofocus, however, can introduce sharp discontinuities in the magnitude and phase information at the edges of the patches. This can introduce potential difficulties in using the data for techniques that exploit phase information such as Coherent Change Detection (CCD) and interferometric elevation extraction.